The invention relates to the field of charged-particle beam systems, and in particular to the design and implementation of high-brightness, space-charge-dominated charged-particle beams, including beam generation, acceleration, focusing, and collection.
High-brightness, space-charge-dominated charged-particle beams are of great interest because of their applications in particle accelerators, medical applications, vacuum electron devices, and material processing such as ion implantation. When the beam brightness increases, the beam becomes space-charge-dominated. In the space-charge-dominated regime, the beam equilibrium is characterized by a beam core with a transversely uniform density distribution and a sharp edge where the beam density falls rapidly to zero in a few Debye lengths.
For particle accelerators, high-brightness, space-charge-dominated charged-particle beams provide high beam intensities. For medical accelerators, they provide high radiation dosage. For vacuum electron devices, they permit high-efficiency, low noise operation with depressed collectors. For ion implantation, they improve deposition uniformity and speed.
However, there are significant theoretical, design and experimental challenges in the generation, acceleration, focusing, and collection of high-brightness charged-particle beams. The traditional approach to charged-particle dynamics problems involves extensive numerical optimization over the space of initial and boundary conditions in order to obtain desired charged-particle trajectories. The traditional approach is numerically cumbersome and will not obtain a global-optimum solution. As a result, beam systems designed using these approaches will result in a degradation of beam brightness, increased noise, particle loss, and reduced efficiency.
An essential component of charged-particle beam systems is the beam generation and acceleration diode, consisting of a charged-particle emitter and an electrostatic gap across which one or more electrostatic potential differences are maintained. The potential differences accelerate the emitted charged particles, forming a beam which exits the diode through an aperture and then enters a beam transport tunnel. Conventionally Pierce type diodes with or without a grid are employed to produce to the Child-Langmuir emission. Compression is often used in Pierce type diodes in order to generate the desired beam radius. Scrapers are also often used to chop off the nonuniform beam edges. The grid, compression and scrapers introduce a mismatch into the beam systems and degrade beam brightness.
A second essential component of charged-particle beam systems is the transition from the diode to the beam focusing tunnel. In the beam focusing tunnel, combinations of magnetic and electrostatic fields are used to confine a beam such that it maintains laminar flow. If the proper focusing field structure is not applied, the beam can undergo envelope oscillations which contribute to beam brightness degradation and particle loss. While the rigid-rotor equilibrium is well-known for a uniform solenoidal focusing field, a good matching of a circular beam from a Pierce type diode into the rigid-rotor equilibrium has not been reported until this invention.
A third essential component of many charged-particle beam systems is the depressed collector placed at the end of the beam transport tunnel to collect the remaining energy in the beam. A well-designed depressed collector minimizes the waste heat generated by the impacting beam while maximizing the electrical energy recovered from said beam. Modern high-efficiency multiple-stage depressed collectors (complicated structures with multiple electrodes held at different potentials) can obtain collection efficiencies approaching 90%.